Drug use is steadily dropping among US teens, suggests a federal survey of 45,000 high school students.

The survey results announced on Tuesday by the National Institute on Drug Abuse (NIDA) focused on drug use among eighth, 10th, and 12th graders in 372 public and private schools nationwide. It found that illicit drug use, besides marijuana, fell among teens in all three grades to the lowest rates ever recorded in the history of the “Monitoring the Future” survey, which started in 1975.

“What we had predicted 15 years ago was that [teen’s drug use] would stabilize, and yet every year they have continued to decrease,” Nora Volkow, NIDA’s director, told BuzzFeed News. “Heroin, cocaine, meth, inhalants, ketamine — all of these drugs, the rate of use is at its lowest it’s ever been.”

While marijuana use among eighth graders dropped in the last year to 9.6%, compared to 11.8% last year, it stayed stable among 10th and 12th graders. Nearly 33% of high school seniors reported smoking marijuana in the last year, and almost 6% reported that they smoked daily. The rate was higher for those in states with medical marijuana laws, with roughly 38% smoking weed in the past year.

Cigarette smoking also dropped in all three grades from the peak in the 1990’s: Almost 11% of high school seniors smoked more than half a pack of cigarettes a day in 1991. That rate is now down to just under 2%.

Similarly, just over half of 12th graders reported drinking alcohol in 2016, compared to the peak rate of around 75% in 1997. In the past year, 38% of 10th graders and 17.5% of eighth graders reported drinking. That is a drop from a peak of 65.3% in 2000 for 10th graders, and 46.8% in 1994 for eighth graders.

Surprisingly, the declines come amidst a painkiller and heroin crisis that’s raging among young adults. Last week, the Centers for Disease Control announced that more than 52,000 people died of overdoses last year — higher than any year on record — largely due to the spike in heroin and prescription opioid use.

“We don’t see these good numbers that we are observing in teens, in adults,” Volkow said. “Something is delaying it in teenagers, but whatever is protective is no longer relevant as teens go into college.”

The NIDA study saw a 45% decrease in opioid use among 12th graders in the last five years, with only 2.9% reporting recreational use of the widely abused painkiller Vicodin, for example, in the last year. That compares to nearly 10%, a decade ago.

One possible explanation for the teen to twenties mismatch is that the NIDA survey is looking at the wrong teens, Suniya Luthar, professor of psychology at Arizona State University, told BuzzFeed News.

Luthar studies elevated levels of drug use among teens in affluent school districts. Those schools are often reticent to participate in national surveys, she said, since they might reflect poorly on their students. As the demographics of the opioid epidemic shift to a younger, whiter, and more suburban population, the schools that NIDA manages to include in the survey may skew the results of the annual survey, if they miss those more affluent ones.

“We’re not capturing the exact trends, because the kids who are using more frequently are not necessarily being included in these large surveys,” Luthar said.

Volkow argued instead that a drop in cigarette smoking, due in large part to successful prevention campaigns, may have led to lower rates of compulsive drug-seeking behaviors overall in teens. She also speculated that teens interacting more through social media has led to less in-person socializing, and therefore less drug use, and that videogames are possibly substituting for drugs as another compulsive behavior.

“These are all very speculative [ideas],” Volkow noted. “These are things we really need to investigate further.”